PROTECTION CIRCUIT FOR ELECTRONIC SYSTEMS IN GENERAL AND FOR AN ELECTRONIC BALLAST OPERATING DISCHARGE LAMPS IN PARTICULAR

Abstract: A protection circuit for an electronic system in general, and an electronic ballast in particular, wherein behind a diode bridge a controllable resistor, formed by a MOSFET, is placed, which will be gradually turned on when the mains voltage is switched on to prevent a high inrush current. The MOSFET will be controlled by a transistor, which determines by the height of the mains voltage, if the MOSFET will conduct. This will protect the electronic system against a high mains voltage.

Description: Protection circuit for electronic systems in general and for an electronic ballast operating discharge lamps in particular. The invention relates to a protection circuit, which takes care of an overvoltage detection and overvoltage protection, and in particular the limitation of the inrush current of an electronic ballast for discharge lamps. A ballast for operating discharge tubes is common knowled¬ ge, in which the current is limited through the discharge lamp, f.i. a fluorescent lamp, by using a choke in serial connection with the lamp. Ingnition of the light usually happens with the aid of a starter, and this starter is in serial connection with the electrodes of the fluorescent lamp. An electronic ballast is a common known circuit with as object to replace the circuit mentioned in de previous alinea. This is achieved by placing two in serial connected switches across the input DC voltage, which one at a time is closed controlled by a predefined control frequency. The disadvantages of known electronic ballast are: 1) A high inrush current as a result of charging a large capacity coursing a peak load in the mains supply. It applies particularly for an electronic ballast that the number of ballasts connected to a mains distribution group is limited. 2) No adequate, non-destructive detection of, and protection against continuous exposure to a high supply voltages. The object of this invention is to provide a protection circuit for electronic ballast in particular,which eli ates the disadvantages as mentioned above, and where the functions, overvoltage detection, overvoltage protection, and limitation of the inrush current are implemented by one simple circuit. A circuit with the characteristics in the head of conclusie 1 are known of e.g. EP-A1-0272514. The previous mentioned goals of the invention are achieved by a protection circuit with the following characteristics The advantages of the invention are: 1) Through the detection of the mains voltage, the limitation of the inrush current will be activated if the mains voltage is correct. The electronic ballast will not be activated as long as the mains voltage is too high. This is a non-destructive method to protect the electronic ballast against a too high supply voltage. 2) The limitation of the inrush current will only be activated in case of a proper supply voltage and the mains will not suffer from any peak loads. Because of this, it will be possible to connect more electronic ballasts to one distribution group under simular circumstances. 3) Another advantage is the definable hysteresis of the overvoltage detection at variable voltage. As a result, small changes in the supply voltage at the switching point will not effect the state of the overvoltage detection. The invention will be further explained by means of attached illustration, figure 1. The overvoltage detection is formed by resistors R25, R26, R27, capacitor C20, and transistor T3; preceding by a diode- bridge with capacitor C4 and an input filter between mains and the diode-bridge. C20 is charged through R25 as soon as the mains voltage is switched on. The voltage at C20 is limited by the voltage divider R25 in serial with R26 and R27. These last two resistors serve as a voltage divider, which control the base of T3 at a high voltage. The mains voltage's switch off value is set through this voltage divider. T3 will conduct if the voltage of C20 crosses a predefined value. As a result controllable resistor MOSFET Q6 remains in a high impedancy state, or reaches this state by arising mains voltage. The inrush current limitation circuit is formed by resistors R28, R29, R30; capacitors C21, C22, C23; zener diodes Dll and D13; and MOSFET Q6. The inrush current limitation is based on the principle that the MOSFET Q6 is opened slowly, therefore peak currents cannot occur. The inrush current limitation circuit is out of function for as long as T3 conducts. This means that the variable resistor, formed by Q6, is in a high impedancy state. The inrush current limitation circuit will be active for as long as T3 is blocked, but only if the present voltage is correct. R20 will charge 021 till Dll's zener voltage is reached. 022 is charged as soon as 020's voltage oversteps D13's zener voltage. The voltage of 022 forms the gate-source voltage of FET Q6 for as long as Q6 does not conduct. At the moment Q6 starts to conduct, the voltage drop of R30 combined with 022, reduces the gate-source voltage of the MOSFET. This means that the channel resistance of the MOSFET decreases slowly and that the current through the MOSFET is limited. R30 takes care of the feed back of the current through the MOSFET in combination with 022. However, 022 will still be charged through zener diode D13, so that the gate-source voltage still increases, and this has as result that the MOSFET reaches a low impedancy state. If the overvoltage gets enabled, 021 will be discharged through T3 and the current through R28 can no longer charge 021. 022 is discharged through R29, putting MOSFET Q6 in a high impedancy state. This means that the connected circuit is no longer supplied through the mains. Further, 023 is used for the suppression of the induction voltage proceeded from the connected switched power supply if the MOSFET Q6 is in a high impedancy state. The overvoltage detection has a certain hysteresis around the switch off voltage. This means that the overvoltage detection will switch off above a definable voltage of supply line V2 and will switch on below a definable voltage of supply line VI, where VI is less than V2. The advantage is that the circuit switches on and off after a clear and a significant change in the mains voltage. The operation of the hysteresis is based on the fact that the average voltage is higher over 04 when the MOSFET bloks compared to when the MOSFET conducts.

Claims: Conclusions: 1) Protection circuit for an electronic device in general, and an electronic ballast in particular, which is placed directly after the diode bridge, and consist of an controllable resistance vormed by a MOSFET, which will be turned on gradually when the mains voltage is switched on, characterised by the MOSFET's control voltage which is vormed by the difference between the output voltage of a, at the input connected switching device, delay netwerk and the feedback voltage over a resistor placed in series with the MOSFET in de main current. 2) Protection circuit in accordance with conclusion 1, charaterised by the MOSFET which is controlled by a transistor which determines, based on the value from the mains voltage, whether the previous mentioned switching of the MOSFET will taken place or not. 3) Protection circuit in accordance with conclusion 2, charaterised by the transistor's control takes place, based on the value of the mains voltage and in case the mains voltage is too high the MOSFET will not be turned on, so the connected circuit is protected against the high main voltage. 4) Protection circuit in accordance with one of the conclusi- ons 2 through 3, charaterised by the MOSFET which will be blocked in case the mains voltage is above a predefined value. The MOSFET will conduct if the mains voltage is below a certain value, lower than the mains voltage, at which the MOSFET will be blocked. If the mains voltage shows small fluctuations around the switching point, the switching condition stays preserved.

Inventors: COSTER PAUL ADRIAAN, COSTER, PAUL, ADRIAAN
Applicants: CM PERSONNEL PARTICIPATION BV, COSTER PAUL ADRIAAN, C.M. PERSONNEL PARTICIPATION B.V., COSTER, PAUL, ADRIAAN
Application year: 1994
Main classification: H02H9
IPC codes: H02H9, H05B41